The present invention relates to a means of driving a power tool and the position of this means within the power tool and in particular a power module and an electric motor for driving a belt sander and the position of the power module and the electric motor in relation to the sandpaper belt of the belt sander.
Sandpaper is used for the removal of surface layers like, for example, a layer of varnish on a piece of wood. A piece of sandpaper may be used manually, which involves the user repeatedly rubbing the sandpaper against the layer of varnish to be removed and the abrasive nature of the sandpaper steadily removing this surface layer. The user will cease the rubbing action once satisfied that the layer of varnish has been removed, thus exposing a clean piece of wood from underneath the varnish.
Manual usage of sandpaper allows the user access to tight corners, however it may also involve a lot of time and significant effort on the part of the user. This time and effort increases with the size of the task and many would agree that the removal of a layer of varnish from the wooden floor of a room in a typical house would be too onerous a task to be attempted by manual use of sandpaper. However, a power tool in the form of an electric sander, using electrical power to drive the rubbing motion of the sandpaper against the surface layer to be removed, would complete such a task more quickly and with significantly less physical effort on the part of the user.
An electric sander uses domestic mains electrical supply or battery electrical supply to drive an electric motor, which in turn drives a mechanism capable of converting the motor""s rotational motion into sandpaper rubbing motion. Sandpaper rubbing motion typically takes one of two forms.
Substantially constant flat linear motion moving relative to the stationary surface layer to be removed, as achieved by a continuous sandpaper belt with abrasive surface on the exterior, rotating quickly in the form of a flat loop about a first driven roller and a second non-driven roller, the rollers being parallel to each other.
Vibrating movement within a flat plane thus quickly moving the abrasive side of the flat sandpaper back and forth against the surface layer to be removed.
Electric sanders may embody either of the above methods of sandpaper rubbing motion depending on the manufacturing cost of the electric sander and the scale of its intended purpose. When designing an electric sander consideration must also be paid to its shape, size and ergonomics. The shape of the electric sander""s body in relation to its sanding surface will influence the electric sander""s ability to reach edges and tight corners, something which is not a consideration when manually using sandpaper. An electric sander employing the rubbing motion as described in (a) above is called a belt sander.
A conventional belt sander typically comprises a main body element having a handle with an electrical switch and containing an electric motor, a driving mechanism, a driven roller, a non-driven roller, and a sandpaper belt, the sandpaper belt being located on the underside of the body element and held in a flat loop by the two rollers. The rollers are connected to the body element and the driven roller is rotatably driven by the electric motor via the driving mechanism, and both the electric motor and driving mechanism are located within or attached to the body element. Some electric motors, like for example a universal motor, may be powered by a domestic mains electrical supply or battery electrical supply. Other electric motors require a power module to convert a domestic mains electrical supply or battery electrical supply into a more suitable electrical supply. The choice of motor and hence the requirement of a power module depends on the desired performance of the belt sander. If a power module is required, it is normally located in the body element of a conventional belt sander and may be powered by domestic mains electrical supply or battery electrical supply.
Typically a conventional belt sander transfers the rotational motion of the electric motor to the driven roller via a driving mechanism comprising a toothed belt and two toothed wheels, arranged in the form of a pulley system. The first toothed wheel is attached to, and rotated by, the electric motor, thereby turning the toothed belt. The toothed belt passes by the side of the sandpaper belt and turns the second toothed wheel which is attached to and rotates the driven roller. This transfer of rotational motion from the electric motor to the driven roller urges the sandpaper belt to turn about the two rollers in the shape of a flat loop, the flat lower exterior face of the sandpaper acting as an abrasive wall against the work surface.
The operation of a belt sander to polish, clean or remove the surface of materials can be hazardous due to the abrasive nature of the sandpaper belt and the rapid speed at which it travels. The user must take care to avoid any contact with the moving sandpaper belt, but the risk of injury can be reduced by a body element which encloses all moving parts except for the sandpaper belt. The toothed belt passes by the side of the sandpaper belt and must therefore extend the overall width of a conventional belt sander. For the sake of safety the toothed belt and wheels are enclosed by part of the body element which will consequently protrude beyond the width of the sandpaper belt if it is to accommodate the toothed belt and wheels. The additional protruding width of the body element inhibits a conventional belt sander from reaching edges and tight corners on the side of the protrusion, thereby occasionally requiring the user to rotate the belt sander through 180xc2x0 in order to use the side of the belt sander on which the body element is substantially in line with the edge of the sandpaper belt. Furthermore, the additional protruding width limits the choice of aesthetic and ergonomic designs that can be applied to the body element of a conventional belt sander.
One aspect of the present invention embodies a new design of belt sander which makes use of the area located within the confines of the sandpaper belt by substituting a normal driven roller for a roller comprising an electric motor. The electric motor is located inside the roller and provides the means for driving the roller. Preferably the electric motor forms the driven roller, thus obviating the need for an additional driving mechanism such as the pulley system characterised by a toothed belt and wheels. In absence of the toothed belt and wheels the width of the belt sander body element may be reduced to no more than the width of the sandpaper belt plus the necessary means for attaching the rollers and other components located within the sandpaper belt to the body element.
The construction of electric motors is a precise task that may involve many different components, some of which are complicated to make. Electric motors like, for example, an induction motor may comprise a multiple-lamination steel rotor and a stator further comprising a complicated field coil, both of which can be a time consuming and therefore costly to manufacture. With the present invention the preferred choice of electric motor is a claw pole motor comprising an internal stator and an external rotor. The stator comprises at least one claw pole stator element and the rotor comprises at least one permanent magnet acting as a magnetic pole. The preferred choice of stator comprises three claw pole stator elements but, as would be apparent to the skilled person in the art, any number of claw pole stator elements may be employed, the number depending on, amongst other things, the available space and the type of power supply. Preferably the rotor comprises a plurality of permanent magnets and the preferred type of permanent magnet is a rare earth sintered magnet. The rare earth sintered magnet gives the advantage of greater flux density per unit volume in comparison to conventional permanent magnets, however other types of permanent magnet may also be used. Assembly of the components forming the claw pole motor is not complicated although this should also be done in a precise manner so that the finished motor functions correctly. A claw pole stator element forming part of the stator of the claw pole motor is constructed from a relatively low number of individual components when compared to other electric motors like, for example, an induction motor. One claw pole stator element comprises two identical and reversed half-claw members and a field coil. The field coil is formed by a simple hoop shaped coil of insulated wire which is considerably less complicated to manufacture than, for example, a field coil directly wound around the teeth of an induction motor""s stator. The half-claw members may be made of mild steel or other ferromagnetic material. Preferably the half-claw members are made of an isotropic soft iron powder composite which is formed by a bonding process to produce a finished half-claw member made to suitably high tolerances such that no further machining or profiling is required before assembly. Collectively these advantages result in a claw pole motor that is inexpensive to build due to its low number of components and simple construction as well as being well suited for this type of use in a power tool.
An alternating magnetic field within a ferromagnetic body like, for example, the solid steel structure of a rotor or stator gives rise to eddy currents and other iron losses which result in the by-product of heat. Unless this production of heat can be reduced to a point where sufficient heat dissipation naturally occurs via its external components, an electric motor will need to be ventilated in order to cool it to an acceptable operating temperature. Furthermore, many electric motors comprise a commutator and carbon brush arrangement to transmit an electrical supply to the field coil of the rotor. Over time wear between the commutator and the carbon brushes results in a carbon dust that must be expelled from inside the motor to prevent malfunctioning caused by excessive carbon deposits. However, power tools operate in a dusty environment and it is also highly desirable to shield a power tool""s internal moving parts from external dust so as to reduce wear and, prolong their working life. With the present invention, the rotor of the claw pole motor produces significantly less heat than an equivalent wound field rotor due to the absence of alternating magnetic flux within its permanent magnets and the attendant electrical losses. Additionally, the isotropic nature of the soft iron composite used to construct the half-claw members means that any heat that is produced within the claw pole motor may dissipate equally and in all directions. Furthermore, permanent magnets do not need an external electrical supply and so a commutator with carbon brushes is not necessary. Absence of carbon brushes and the resulting carbon dust as well as less heat production means that the claw pole motor, as according to this invention, may be of a shielded construction because internal ventilation is not necessary.
Another aspect of the present invention embodies a new design of belt sander which makes use of the area within the confines of the sandpaper belt by relocating the power module from inside the body element to within a casing, the casing being located in the space between the driven roller and the non-driven roller. This space is within the confines of the belt and is typically reserved for the belt tension adjuster alone in a conventional belt sander. The casing may additionally provide a location for a battery should the battery be the power module""s source of electrical supply. Alternatively, the casing may provide a location for a battery in substitution for the power module should the electric motor be powered directly by the battery without the need for a power module. For safety reasons a belt sander, having a power module, encloses the power module in a protective casing so as to shield the user from the electrical current supplied to its components. However, these electrical currents produce heat as they flow through the components of the power module and this heat needs to be expelled otherwise the power module will overheat. The power module of a conventional belt sander is normally located within the body element which acts as a barrier to efficient heat transfer between the power module, its casing and the surrounding atmosphere. The present invention overcomes this limitation by locating the casing in the space between the driven and the non-driven rollers, this space being exposed to the atmosphere. The heat produced by the components of the power module may be transferred to an internal heat sink, the heat sink being thermally coupled to the casing so that the surface area of the casing behaves as an extension to the heat sink, thereby adding to the cooling capacity of the heat sink. This additional cooling capacity increases the rate of heat transfer from the components of the power module to the atmosphere surrounding the casing. Therefore a power module located within an external casing, as according to the present invention, is more efficiently cooled than a power module located within the body element of a conventional belt sander.
The relocation of the electric motor and the casing for the power module from within the body element to the space enclosed by the sandpaper belt is a more economic use of this space and may result in a more compact belt sander. Consequently the body element simply provides a location for the electrical switch and forms a handle to be grasped by the user because it no longer needs to accommodate any major internal components. This allows more scope for alternative styles of belt sander which may be smaller or more aesthetically pleasing to the user or purchaser.
Accordingly the present invention provides for a power tool comprising a body, a motor and, a roller, characterised in that the motor acts as the roller.
Preferably the motor is an electric motor having a stator and rotor, wherein the rotor is located outside the stator and is capable of rotating about the stator.
Preferably the rotor is the roller.
Preferably the stator is attached to the body.
Preferably the power tool further comprises a non-driven roller.
Preferably the non-driven roller is rotatably disposed upon an axle, the axle being attached to the body.
Preferably the power tool further comprises a belt, the rotor and the non-driven roller being capable of supporting the belt.
Preferably the rotor comprises a cylindrical drum and a plurality of permanent magnets, the permanent magnets being attached to the inside of the cylindrical drum.
Preferably the permanent magnets are sintered rare earth magnets.
Preferably the motor is a brushless shielded motor.
Preferably the stator is a claw pole stator comprising at least one claw pole stator element.
Preferably a claw pole stator element comprises a field coil, a first half-claw member and a second half-claw member, the first half-claw member comprising a first central element and a plurality of claws, the claws being arranged in equi-angular intervals around the perimeter of the first half-claw member, and the second half-claw member comprising a second central element and a plurality of claws, the claws being arranged in equi-angular intervals around the perimeter of the second half-claw member, wherein the claw pole stator element is formed when the first half claw member and the second half claw member are joined at the first central element and the second central element thereby causing the claws to juxtapose about the perimeter of the first half-claw member and the second half-claw member, the claws enclosing the field coil and, the field coil surrounding the joined first central element and second central element.
Preferably the first half-claw member and the second half-claw member are formed of an isotropic ferromagnetic composite material.
Preferably the claw pole stator further comprises a shaft and a plurality of claw pole stator elements the claw pole stator elements each concentrically disposed upon the shaft.
Preferably the shaft is formed of a non-magnetic material.
Additionally or alternatively the stator comprises a laminated core having a plurality of laminated teeth, a field coil and, a shaft, the laminated core being fixedly secured upon the shaft.